1. Field of the Invention
The present invention relates to a field of an optical transmission-reception apparatus used in a single-core bi-directional optical communication circuit using an optical fiber and more particularly, to an optical transmission-reception apparatus for use in an optical communication circuit using a plastic optical fiber in a domestic communication network.
2. Description of the Prior Art
There has been proposed an optical communication circuit connected to an optical transmission-reception apparatus. The optical transmission-reception apparatus constituting such an optical communication circuit is used for connection between various apparatuses such as a computer, a digital video apparatus, and an audio apparatus.
FIG. 1 shows a conventional optical transmission-reception apparatus including a socket 104 to be connected to a plug 103 holding a first plastic optical fiber 101 for transmission and a second plastic optical fiber 102 for reception. In this optical transmission-reception apparatus, the socket 104 has a built-in light-emitting element 105 for emitting a first optical signal 106 which has been modulated according to a signal to be transmitted and a built-in light-receiving element 108 for receiving a second optical signal 107 which has been modulated according to a transmitted signal. The light-emitting element 105 and the light-receiving element 108 are divided from each other by a shielding wall 109 to prevent leaking of the first optical signal 106 and the second optical signal 107 into the light-receiving element 108 and the light-emitting element 105, respectively.
In a state when the plug 103 is connected to the socket 104, the first optical signal 106 emitted from the light-emitting element 105 is introduced into an end face of the first plastic optical fiber 101. Moreover, in the state when the plug 103 is connected to the socket 104, the second optical signal 107 emitted from an end face of the plastic optical fiber 102 is received by the light-receiving element 108.
In an optical communication circuit constituted by using the aforementioned transmission-reception apparatus, two plastic optical fibers are used: one for transmission and the other for reception. Consequently, the plug and the socket have a considerable size, which is not preferable for an optical communication circuit to be used as a domestic communication network. That is, the optical communication circuit to be used as a domestic communication network should be simplified as a single-core bi-directional optical communication circuit.
In an optical transmission-reception apparatus constituting a single-core bi-directional optical communication circuit, it is necessary to coaxially overlap the first optical signal which is a transmission signal and the second optical signal which is a reception signal. As means for coaxially overlapping two beams having opposite advancing directions, there can be considered use of a so-called half mirror (semitransparent film). That is, one of the first and the second optical signals has an optical path passing through the half mirror and the other of the optical signals has an optical path reflected by the half mirror. Thus, the first optical signal and the second optical signal are coaxially overlapped.
However, in this half mirror, the total of the transmittance and the reflectance does not exceed 100% and each of the transmittance and the reflectance is equal to or less than 50%. That is, when overlapping the first optical signal and the second optical signal by using a half mirror, the optical signal passing through this half mirror and the optical signal reflected by this half mirror have a loss of about 50%, respectively.
If the first optical signal which is a transmission signal is decreased in light quantity and introduced into the plastic optical fiber, the transmission distance available is reduced and it is impossible to assure a sufficient performance and reliability of the optical communication circuit.
In order to compensate the loss of light quantity because of the half mirror, it can be considered to increase the light quantity emitted by the light-emitting element. However, in a laser diode used as the light-emitting element, the light quantity emitted has a trade-off relationship with the service life and reliability of the element. If the light quantity emitted is increased, then the service life of the element is reduced and its reliability is lowered.